Separate round cube in blender 2.8
Your question is not clear, if you're talking about what he's doing a 3:10 (not sure though):


Select the 3 edges of a corner, then press shiftG (select similar) > Length, it will select all the edge with a similar length, in this case the edges separating the 6 faces of the cube.
Now to separate these 6 faces, just press V (rip).
As Pivot Center, choose Individual Origins, now when you scale down the whole mesh, each face will scale individually.
Note that his cube is shrinkwraped to another round cube, which makes the individual faces follow a round profile.